Air Force files from a 1952 incident in which a Salem man captured photographs of four bright lights in the sky, which might have been the reflection of lamps in a window.

WASHINGTON — James Louis Warsher and a friend were driving across the Anderson Memorial Bridge in Harvard Square when they noticed a dull gray elliptical vessel tinted with muted yellow light descending from the sky. It was around 8:30 p.m. on Sept. 9, 1964, and the two were among 11 Bostonians who reported UFO sightings that night.

Air Force investigators said the object was just an advertising plane. They confirmed it by cross-checking the aircraft’s flight plan with civilian reports of when and where they saw UFO.

But the Air Force’s official explanation — here you may cue the “X-Files’’ theme song — couldn’t answer one issue: When Warsher and his friend saw the UFO, it was flying within sight of the same plane the investigators purported it to be.

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This story is one of more than 12,000 chronicled by Project Blue Book, the Air Force’s UFO investigation program that ran from 1947 to 1969. The once-classified files spent most of the past 40 years as microfilm reels in the National Archives, but they have become more accessible to the general public in recent years through Fold3, a commercial database of government and military documents.

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The database includes 4,429 reports from New England — 1,910 from Massachusetts — though others from the region might not be properly sorted. Readers can sort the files by date, state, and when they were added to the website.

The pages of witness testimonies, official memos, maps, and flight patterns bring to life a pivotal period in American history, when a wealthier but war-shaken United States turned its attention to technological threats from the Soviet Union and beyond. The collection reveals how the nation’s military bureaucracy painstakingly documented what amounted to a cultural phenomenon of the Cold War.

“It’s a part of America’s comparatively recent history that people have forgotten, if indeed they ever knew it,” said Nick Pope, a California researcher and freelance journalist who investigated UFO sightings in the United Kingdom for the British government. “For years, actually, the government took this very seriously, and I think people quite often are skeptical and cynical about UFOs, probably because of the treatment it gets from Hollywood or even shows like ‘South Park.’ ”

A team of Air Force investigators conducted Project Blue Book out of Wright-Patterson Air Force Base in Ohio, taking in reports of UFO sightings from across the country and churning out explanations for what civilians and military personnel witnessed. Investigators dismissed the vast majority of these cases as aircraft, balloons, weather inversions, or heavenly bodies.

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Project Blue Book was conducted in an era defined by technological innovation and immense paranoia. It ran from two years after the atomic bombings in Japan until the year men walked on the moon, and peaked with the tensest years of the Cold War. With Americans on the lookout for Soviet or Martian invaders, recorded sightings range from innocent mistakes by well-meaning people to bizarre reports from military officials.

The military preoccupation with both UFOs and covert planes has roots in a pre-World War II effort to bolster aerial photography, said Donald Welzenbach, a retired CIA officer who co-wrote a 1998 report that attributed many UFO sightings to American U-2 spy planes.

“All of these little people you don’t know about,” he said, “these guys were brilliant.”

Not every alleged UFO sighting was secret military technology. A Salem man in 1952 captured stunning photographs of four bright lights in the sky, which might have been the reflection of lamps in a window. In 1965, a man near Pease Air Force Base in Portsmouth, N.H., mistook a NASA satellite for an alien spaceship.

Other reports are still puzzling. On Feb. 5, 1950, seven people in Teaticket, a community on Cape Cod, saw two cylindrical objects float in the sky one evening. One of the objects dropped a fireball, and the pair quickly vanished. Seven months later, two F-86 pilots flying out of Otis Air Force Base picked up an aircraft on their radar going 1,200 miles per hour. The pilots never saw it, and Air Force personnel said the mystery craft made a turn so tight at a speed so high, the pilot would have endured forces sufficient to render some people unconscious.

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Even though the Air Force deemed only a small portion of the reports unexplainable, UFO researcher Stanton Friedman believes Project Blue Book barely scratches the surface of American UFO investigations. Friedman claims “the Air Force has gotten away with carefully worded misrepresentation,” of its interest in the aircraft.

The Air Force, which declined to comment for this article, shut down Project Blue Book in 1969 after the agency found no threats to national security, no novel technology, and no evidence of extraterrestrial origin from the unknown aircraft. The Red Scare is history, and scientists have now turned to distant icy moons in the search for extraterrestrial life. Still, the unanswered queries from a bygone era linger.

“I wish the government would get back into this,” said Pope, “because it’s one of the biggest and most profound questions of all time.’’